The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the adaptive averaging of real-time velocity spectra using variable density trajectories.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Cardiovascular diseases that restrict blood flow, such as valvular stenosis and aortic coarcation, can be assessed with measurements of the peak velocity of blood through the constriction. Real-time MR measurements of the velocity spectrum of blood are possible using two dimensional (xe2x80x9c2Dxe2x80x9d) selective excitations with one dimensional (xe2x80x9c1Dxe2x80x9d) Fourier velocity encoding (xe2x80x9cFVExe2x80x9d). However, there is an inherent trade-off between the velocity resolution, aliasing velocity, and temporal resolution of such measurements. To improve the velocity resolution without changing the temporal resolution, variable-density (xe2x80x9cVDxe2x80x9d) trajectories can be used to sample edges of velocity k-space more coarsely than the center. A drawback of a VD approach is that it will introduce aliasing artifacts when the velocity spectrum contains significant high-frequency information (e.g., plug flow). To overcome this problem, an embodiment of the present invention provides for the adaptive averaging of real-time velocity spectra using variable density trajectories.
A system and method for adaptive averaging of velocity spectra using variable density trajectories, comprising: acquiring at least one series of velocity spectra using interleaved variable density trajectories and sampling low kv data more often or more densely than high kv data, the series of velocity spectra further comprising at least one spectrum; identifying a series of velocity spectra that comprises at least one velocity spectrum as a template, aligning at least one of the acquired series of velocity spectra with the template using low kv data; and averaging the aligned spectra, the averaging further comprising averaging the low kv data of the aligned spectra, and combining the averaged low kv data with the high kv data of the aligned spectra, wherein the combination produces enhanced spectra.